1. Field of the Invention
The present invention relates generally to the field of semiconductor manufacturing and, more particularly, to a device and method for in-situ identification and quantification of elements having a higher atomic weight that the elements of a workpiece for on-line diagnostics of purity and content of the substrate.
2. Description of the Prior Art
The ability to routinely measure very small levels of contaminant elements in a workpiece is increasingly important in the area of microelectronics as integrated circuit device sizes become ever smaller. In the microelectronics area, for example, as few as 10.sup.11 -10.sup.12 /cm.sup.2 of heavy metal contaminant elements in a silicon workpiece can create major problems with the gate oxide in manufactured MOS devices by greatly reducing the oxide breakdown strength.
Devices are available for identifying unknown contaminant elements in a workpiece. One device, which is used extensively, is the Rutherford Backscattering Spectrometry (RBS) device which detects and identifies unknown contaminant elements in a workpiece having a mass greater than the mass of the primary element forming the workpiece. The RBS device employs a high energy ion beam (typically a one to two MeV He.sup.+ ion beam) that impacts the surface of a workpiece. The ions from the beam collide with the atoms near the surface of the workpiece and are scattered with a measurable energy and momentum. A portion of the backscattered high energy ion beam is directed towards a detector. The detector counts the backscattered ions and measures their energies which enables the identification and quantification of contaminant elements present in the workpiece.
Although the Rutherford Backscattering Spectrometry device provides one means for identifying unknown contaminant elements in a workpiece, it has major limitations. One limitation of the RBS device is that the use of ion beams at high energies, 1 to 2 MeV, eventually damages the detector.
Another limitation is its sensitivity or the ability to detect small concentrations of the unknown contaminant elements present in the workpiece. The small cross sections (typically less than 0.1 to 10 barns) for scattering from the unknown contaminants coupled with the interference of background noise introduced at the detector because of pulse pileup preclude greater sensitivity. For example, a none to two MeV He.sup.+ ion beam used according to the principles of the RBS device to analyze a silicon workpiece has an average sensitivity of approximately 10.sup.13 atoms/cm.sup.2.
The sensitivity of a RBS device may be increased by increasing the cross-section for scattering where the cross-section is a parameter of probability of a particular process, in this case, a collision between particles in the ion beam and atoms in the workpiece. The cross-section for scattering is proportional to (Z.sub.1 E.sub.0).sup.2, where Z.sub.1 is the atomic number of the ion beam and E.sub.0 is the incident energy of the ion beam. Thus, increasing the cross-section may be accomplished either by decreasing the energy level of the ion beam, increasing the atomic number of the ion beam, or by doing both (using a heavier ion beam at a lower energy level to impact the surface of a workpiece). For example, using a four hundred (400) KeV C.sup.+ ion beam instead of a two (2) MeV He.sup.+ ion beam increases the sensitivity approximately two hundred and twenty five (225) times. Although the sensitivity of the RBS device may be increased by decreasing the energy level of the ion beam or increasing its atomic number, these techniques have not been widely exploited because the increased number of generally lower energy, backscattered ions from the workpiece saturate the detector and preclude detection of ions backscattered from any heavier mass unknown contaminant elements. In order to protect the detector utilized to count ions backscattered from the workpiece and from contaminant element atoms in the "increased sensitivity" situation, both pulse pileup rejection circuitry and a cooling system for the detector are required.
Consequently, a need exists for an improved backscattering spectrometry method and device which, while a workpiece is being implanted with an ion beam, prevents lower energy backscattered ions from the workpiece atoms from saturating the detector to preclude or "overwhelm" the detection of higher energy ions backscattered from unknown contaminant elements. The measurement of the energy and number of these ions enable the identification of the contaminant elements based on their mass. Preferably, the sensitivity of the backscattering spectrometry device should be increased without requiring the use of presently needed pulse pileup rejection circuitry and/or a cooling system for the detector. And ideally, the device and method provides for on-line or in-situ diagnostics wherein the same beam that is used for implantation during the manufacturing of semiconductor devices is used for monitoring the level and kind of contamination.